Heretofore, diesel internal combustion engines mounted on motor-driven vehicles, construction machines and power generators are generally operated using gas oil or heavy oil (which is a fuel having a sulfur content of approximately 0.05 wt. % or more). Most lubricating oils for diesel engines have a sulfated ash content of approx. 1.3 to 2 wt. %, a sulfur content of approx. 0.3 to 0.7 wt. %, a phosphorus content of approx. 0.1 to 0.13 wt. %.
As for internal combustion engine-mounted vehicles, particularly diesel engine-mounted vehicles, it is required to find ways to obviate environmental pollution problems, such as particulates, caused by the exhaust gas components. For obviating such environmental pollution, exhaust gas-cleaning devices containing a particulate filter and oxidizing catalysts are mounted on the vehicles. The combination of the particulate filter and oxidizing catalysts trap the produced soot and then oxidize and burn the trapped soot. However, metal oxides, sulfates and carboxylates produced by burning of conventional lubricating oils are apt to plug the particulate filter.
Sulfur contained in diesel fuels is also converted to sulfuric acid and sulfates which emigrate into the exhaust gas. The sulfuric acid and sulfates poison, i.e., lower activity of, the oxidizing catalysts in the exhaust gas-cleaning device. Accordingly, it is desirable to decrease the sulfur content of diesel fuel. In the near future, it is expected that new requirements will be issued for further decreasing the sulfur content of diesel fuel to approx. 0.001 wt. % or lower from the present value of approx. 0.05 wt. % to 0.01 wt. % or lower.
As the sulfur content of diesel fuel is decreased, the content of the metal-containing detergent (which functions to neutralize the produced sulfuric acid) in the lubricating oil can be decreased. Lubricating oil is employed for lubricating engine parts, but a portion of the lubricating oil is burnt and emigrates into the exhaust gas. Therefore, the decreased metal-containing detergent content, which means a decrease of the metal content and the sulfur content, is favorable for reducing environmental pollution. Moreover, it is preferred to decrease the phosphorus content in the lubricating oil so as to keep the oxidizing catalysts in the exhaust gas-cleaning system from deterioration. It is also desirable that the content of-chlorine in the lubricating oil is also as low as possible, so as to decrease production of dioxine.
U.S. Pat. No. 5,102,566 describes a low sulfated ash lubricating oil composition which comprises a base oil, at least about 2 wt. % of an ashless nitrogen- or ester-containing dispersant, an oil-soluble antioxidant material, and an oil soluble dihydrocarbyl dithiophosphate anti-wear material, and which has a total sulfated ash (SASH) level of 0.01 to 0.6 wt. % and a weight ratio of SASH to the dispersant in the range of 0.01:1 to 0.2:2.
Japanese Patent Provisional Publication No. 8-48989 describes a lubricating oil composition which has a low sulfated ash content and does not disturb the functions of particulate traps and oxidizing catalysts, but shows good stability at high temperatures so that it can meet the anticipated exhaust gas regulations. The disclosed lubricating oil composition comprises at least 5 wt. % of a boron-containing ashless dispersant, 0.05 to 0.15 wt. % (in terms of phosphorus content) of zinc dithiophosphate, and optionally 0.01 to 2 wt. % of an ashless oxidation inhibitor. The boron content in the composition is at least 0.1 wt. %, the boron content/phosphorus content ratio is at least 0.8, and the sulfated ash content is at most 1.0 wt. %.
European Patent Application No. EP-A-0 686 689 A2 describes an internal combustion engine lubricating oil composition for use with a maintenance-free engine system, which has a total base number of 2.0 to 6.0 mg KOH/g, a low sulfated ash content, and a low phosphorus content. The disclosed lubricating oil composition comprises a specific alkaline earth metal type cleaning agent, zinc dialkyldithiophosphate, a succinic acid imide type ashless dispersant and a phenol type and/or amine type ashless antioxidant.
Japanese Patent Provisional Publication No. 2002-53888 discloses a lubricating oil composition having a sulfur content of 0.01 to 0.3 wt. %, a phosphorus content of 0.01 to 0.1 wt. %, and a sulfated ash in the range of 0.1 to 1 wt. %, which is favorably employable in combination with a fuel oil having a low sulfur content and comprises:                a) a major amount of a mineral base oil having a sulfur content of at most 0.1 wt. %;        b) an ashless dispersant comprising an alkenyl- or alkyl-succinimide or a derivative thereof in an amount of 0.01 to 0.3 wt. % in terms of a nitrogen atom content;        c) a metal-containing detergent containing an organic acid metal salt which is selected from the group consisting of a non-sulfurized alkali metal or alkaline earth metal salt of an alkylsalicylic acid having a TBN of 10 to 350 mg KOH/g and a non-sulfurized alkali metal or alkaline earth metal salt of an alkylphenol derivative having a Mannich base structure, in an amount of 0.1 to 1 wt. % in terms of a sulfated ash content;        d) a zinc dialkyldithiophosphate in an amount of 0.01 to 0.1 wt. % in terms of a phosphorus content; and        e) an oxidation inhibitor such as a phenol compound or an amine compound in an amount of 0.01 to 5 wt. %.        